Reinforcement for Slabs on Grade

[Woody1515]

Hello everyone,

I have a very basic question regarding the design of slabs on grade (bear with me). Is it safe to assume the compacted fill underneath will support the slab and prevent it from bending? If so, is any reinforcement in a slab-on-grade primarily there for temperature and shrinkage? If not, what assumptions are made in the flexural design?

 

[Ron]

Yes. Unless the loads are specific, the reinforcement is for temperature.

 

[Woody1515]

Thanks for the fast response. In what cases would you design the slab for flexure if it is on-grade?

 

[jayrod12]

Point loads or line loads directly on the slab, and somehow if you know there's a certain possibility of upwards loading of known magnitude.

 

[jike]

The slab will flex even though it is ground supported as if was supported on a series of springs. There are standard spring constants that can be assumed for various conditions or the soil can be tested to determine the spring constant. Charts/tables are available as guides for recommended slab thicknesses for various applications. Many slabs for commercial or light industrial resist the flexure as unreinforced concrete slabs. WWF or reinforcing is added in the upper third to resist temperature and shrinkage stresses. Some heavy industrial slabs may add reinforcing in two layers to resist flexure from heavy point loads.

 

[Guastavino]

Load goes to stiffness. The soils is a idealized as a giant spring, so any loads that you have will dissipate and spread out over a certain area based on the stiffness of the soil and stiffness of the slab. Much like water flows downhill, load flows with stiffness...so even a residential basement floor slab with walking traffic will experience bending. Albeit, in cases like that the magnitude is so small it's neglected.

That and often times the soil settles underneath slabs and they are only partially bearing anymore anyway...Point is, you need to use engineering judgment on the specific application. There isn't a one-size fits all answer, but ACI 360R-10 is a great resource to help explain